This invention relates to a device for quantifying a useful thermal energy available in a tank for storing a heated or cooled fluid or solid. It also relates to a system for quantifying useful thermal energy comprising such a device.
A tank for storing a heated fluid or solid is generally used in systems for generating, storing and restoring heat. Likewise, a tank for storing a cooled fluid or solid is generally used in systems for generating, storing and restoring cold.
More concretely, a simple and common example of application consists in using a storage tank of heated water in a system for generating and for consuming sanitary hot water or water for heating. In such a storage tank, hot and cold water cohabitate and tend to remain gradually separated into horizontal strata or isotherms in each one of which the water substantially has the same temperature. But from one stratum to the other, the temperature of the water differs, with the strata of hot water being arranged in the upper portion of the tank and the strata of cold water in the lower portion. More precisely, the higher one goes in the succession of strata inside the tank, the hotter the water is. This stratification phenomenon of the water in a hot water tank is well known to those skilled in the art. It is due to the effects of universal gravity on the density of water which is a decreasing function of temperature.
As such, during the heating, in the storage tank, of a fluid initially at a uniform temperature, the stratification of the fluid is established naturally.
Moreover, even when the water heated and stored in the tank is consumed, the stratification is preserved. Indeed, while hot water is generally taken in the upper portion of the tank, cold water enters via the lower portion of the tank in order to then be heated, in this way, the hottest stratum of water always remains at the top of the tank, while the incoming cold water constitutes the coldest stratum of water at the bottom of the tank. Between these two extreme strata are the strata of water being heated, of which the temperatures increase progressively, from the coldest stratum at the bottom to the hottest at the top.
In order to improve the performance of a system for generating and for consuming hot water, in particular in terms of saving energy, it is advantageous to be able to quantify at any time the useful heat available in the storage tank of such a system. This quantity of useful heat is defined a minima as being the quantity of water inside the tank of which the temperature is greater than or equal to a predetermined threshold temperature. The value of this threshold temperature represents the minimum value of the temperature that is required for the use, by a consumer, of the hot water restored by the system. In the case of a system for generating, storing and restoring sanitary hot water for example, this threshold temperature is generally 40° C.
The interest in quantifying the useful thermal energy available in the tank is in particular to be able to act on the regulation of the systems for generating and storing heat or cold, in particular when these systems use a renewable source of energy, for example solar, or when these systems use renewable sources of energy combined with fossil or nuclear sources. In this type of systems indeed, as the supply with energy by the renewable energy source varies substantially, a specific regulation for each situation is required in order to optimise the generating of heat or of cold by soliciting fossil sources as least as possible.
Moreover, currently, in order to be able to cover the needs of users during the periods of peak consumption and without being able to quantify the useful heat available at any time, the systems for producing sanitary hot water are oversized with respect to an average consumption of sanitary hot water evaluated. Of course, this prevents a possible discontent of users due to an occasional rupture in the availability of hot water, but at the price of an increase in the thermal losses of the systems and therefore a decrease in their efficiency or output. Being able to quantify the useful heat available in a tank makes it possible to inform the user on the quantity of hot water remaining in the tank, which allows the user to consequently react and to as such participate in a saving of energy through the knowledge at every instant of the quantity of hot water available. The need to oversize the system for producing the corresponding sanitary hot water is then avoided. In particular, when a value for the quantification of the useful heat indicates that the level of hot water available in the tank is under a predetermined threshold value, a switching to a less expensive economical consumption mode in terms of energy can be considered, in particular the reducing of the consumption of hot water by the user.
More generally, the interest of a device for quantifying useful thermal energy available concerns any industrial system, that has a tank for storing a heated or cooled fluid or solid, of which the performance can be improved thanks to good management of the storage of heat or of cold.
However, it can be difficult to measure the quantity of water contained in the storage tank of which the temperature is greater than or less than a desired threshold temperature. By way of example, in the case where there are two sensors independently measuring water temperatures at the bottom and at the top of the tank, if the two temperatures are substantially equal this implies that the contents of the tank is at a uniform temperature and therefore this temperature is known. On the other hand, if the two temperatures are far apart, it is impossible with only these two measurements to know where in the tank is the limit between the hot water and the cold water and consequently to estimate the quantity of useful heat or cold available in the storage tank.
Patent application published under number EP 2 017 587 A1 discloses a thermomechanical device for the quantification of hot water available in a storage tank and more particularly in a sanitary hot water tank. This device includes a capillary tube containing a heat transfer fluid arranged in or against the storage tank and extending over practically all of its height. The heat transfer fluid dilates under the action of the heat of the water in the storage tank and, in a first embodiment possible, mechanically engages the rotation of a needle by the intermediary of a Bourdon tube. This needle directly indicates the estimated quantity of hot water available in the tank according to the pressure that is exerted against the Bourdon tube. In a second possible embodiment, the dilatation of the heat transfer fluid mechanically engages the deformation of a membrane of an electronic pressure sensor. This sensor transcribes the deformation of the membrane into electrical data, in particular a variation in resistance, which can then be used by an electronic circuit in order to indicate an estimation of the quantity of hot water available.
This device has several disadvantages. Firstly, contrary to what it claims, it does not actually measure the quantity of hot water available in the tank, with the hot water having to be defined with respect to a threshold value as discussed previously. It rather measures the quantity of heat globally available in the tank, without being able to know what proportion of the water is actually “hot”, i.e. what quantity of water has a temperature that exceeds the threshold value. Furthermore, this device is rather complex to set up. In the first embodiment, it requires a thermomechanical system with a capillary tube of heat transfer fluid associated with a Bourdon tube which itself drives a needle. In the second embodiment, it requires a thermomechanical system with a capillary tube of heat transfer fluid associated with an electromechanical system with an electronic pressure sensor, which involves the double conversion of a thermal variable into a mechanical variable then of this mechanical variable into an electrical variable.
A second solution consists in providing several thermoelectric converters distributed at several locations of the storage tank. By providing a sufficient number of thermoelectric converters, it is possible to reach a satisfactory quantification of the hot water present in the tank. By supposing for example that each thermoelectric converter concerns a predetermined volume of water at a constant temperature in the tank, it is easy to deduce from the measurements the quantity of water of which the temperature exceeds the threshold value. Another advantage of this second solution is that it is simple, thanks to a single conversion of a thermal variable into an electrical variable.